String() should return the compact string format by default

[git-hulk]

This closes #89

Currently, we don't have a consistent way to add a new line for printing SQL
and it's hard to follow for now. So I prefer to remove this and bring it back
while we have a clear solution.

[coveralls]

Pull Request Test Coverage Report for Build 10630936931

Details

• 396 of 524 (75.57%) changed or added relevant lines in 1 file are covered.
• 26 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.09%) to 59.3%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
parser/ast.go	396	524	75.57%

Files with Coverage Reduction	New Missed Lines	%
parser/ast.go	26	55.34%

Totals
Change from base Build 10629830518:	-0.09%
Covered Lines:	6131
Relevant Lines:	10339

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💛 - Coveralls